From what I've read the first cities in Egypt, Mesopotamia and Mexico* had populations numbering in the tens of thousands from around 30,000 to 80,000. So I'm picking 50,000 as the size of the biggest cities in a low population scenario or time. The biggest cities during a Classical Age type of development is 1,000,000 equivalent to ancient Rome and Alexandria at the height of the Roman Empire.
I'm imagining one city for each of the big river systems, South, East and West and one for the big inland sea, so four in total. The definition of a city in this model is loose because I take a lot of inspiration from the Maya and Khmer.
To calculate the number I've used the following rule. For each tier of settlement below the biggest city I double the number and halve the population, rounding up to whole numbers. I do this down to around 100 people which corresponds roughly to Dunbar's Number. So if there are 4 cities with an average population of 50,000 then there are 8 cities with 25,000 inhabitants, 16 with 12,500 inhabitants etc.
Depending on the characteristics of the culture and where it's located these numbers might be for a compact city or town, a spread out settlement or even a roaming band of hunter-gatherers.
*Recent research indicates that civilisations in rain forests were able to sustain larger populations than previously thought. See further below for some research about these fascinating urban environments.
Large urban communities in rainforest environments
Recent findings in Mexico and Cambodia indicate that the population that could be sustained in these jungle environments was higher than initially thought (at least for a while). It seems that there once existed sprawling urban communities within the forests. While I'm sure large areas were cleared it is nevertheless an evocative idea that contrasts with the common idea of what a city looks like. The articles linked are short but are great for stimulating the imagination. They also discuss the reasons these civilisations might have collapsed.
In what's being hailed as a 'major breakthrough' in Maya archaeology, researchers have identified the ruins of more than 60,000 houses, palaces, elevated highways, and other human-made features that have been hidden for centuries under the jungles of northern Guatemala.Exclusive: Laser Scans Reveal Maya "Megalopolis" Below Guatemalan Jungle
It also seems that there was more warfare happening amongst the Maya than was initially thought.
Archaeologists guided by laser images of a remote region of northern Guatemala have discovered 20-foot-high walls, watchtowers, and other evidence that ancient Maya societies waged large-scale warfare over many years. The finds have upended long-established impressions of a civilisation that tamed the jungle and built thriving cities, then declined and disappeared beneath the dense tropical forest.Lasers reveal Maya war ruins
Articles about the city around Angkor Watt in Cambodia which had a sophisticated water management system using canals and artificial ponds and lakes.
Angkor was a city ahead of its time
Revealed: Cambodia's vast medieval cities hidden beneath the jungle
An article about Maya ruins uncovered by LIDAR that contains some great images:
How Lasers Are Utterly Transforming Our Understanding of the Ancient Maya, Bringing Their Whole Civilization Back to Light
Mayan Mysteries: Discusses possible reasons for the collapse of the cities including deforestation and drought.